The effects of undernutrition on the "gastrocnemius plantaris" muscle of young albino rats were observed with light and electronmicroscopy and were compared with controls. Pregnant rats received a diet containing 6.7% protein and the neonates had a 3.2% protein diet after weaning. A total number of 40 animals were distributed in two groups: one hypoproteic and one control. Half of the animals of each group were killed 15 days after birth and the remaining ones at 30 days. We could observe important reduction in the weight of the undernourished rats reaching about 50% when compared with the control animals. An important reduction in the diameter of muscle fibers was noted in undernourished rats. Histochemical reactions showed that not only the type I but also the type II fibers were involved, the latter being more severely reduced in size. Type II fibers os small diameter, resembling F (fetal) fiber were found in animals at 15 days indicating a delay in maturation. Ultrastructural evaluation of the muscle did not show specific changes except for a severe reduction in the caliber of fibers when compared with control animals. We concluded that there was hypoplasia and not atrophy of the muscular tissue in animals submitted to pre- and post-natal undernutrition. The present study, associated with others in which the spinal motor cells and peripheral nerves of undernourished rats were analysed, allow us to consider that with protein deprivation there is a delay in the development, a hypoplasia of the motor unit. We think that in infantile progressive spinal amyotrophy (Werdnig-Hoffmann disease) there is possibly hypoplasia and not atrophy of the type II fibers and we postulate that a metabolic proteic problem is involved in this disease.